The present invention relates generally to materials and methods for glazing and sealing a porous ceramic surface. In particular, the invention relates to a glass-based glaze capable of sealing the ends of a porous alumina (Al2O3) ceramic filter tube. This xe2x80x9cmembrane-supportxe2x80x9d tube provides mechanical support for a thin film microporous membrane, which is used to separate hydrocarbon gases at high temperatures. The gas impermeable end seal is used to prevent gas from leaking around the ends during gas separation or other filtration processes.
Porous ceramic filter media are widely used in chemical processing applications, including liquid phase separations, solid/liquid separations, gas separations, contamination control, and membrane reactor applications. Filter media made of ceramics are generally superior to organic or metallic filter media because of their high temperature stability and inherent resistance to chemical attack. The media can take many geometric forms, such as open or closed-end tubes, plates, cylinders, discs, etc.
Microporous thin film membranes are used in chemical and petroleum refining processes for separating mixtures of gases by size exclusion (i.e., molecular sieves), such as separating various vaporized organic hydrocarbon isomers in a feedstock stream, or separating individual light gas molecules in stream. Amorphous silicon (e.g., solgel) membranes can be used for low temperature gas separations. The membrane""s porosity is highly controlled in order to achieve a high flux and high separation efficiency. Microporous membranes are typically deposited or crystallized on a macro-porous mechanical support structure, such as on the inside of a macro-porous alumina tube (hereinafter referred to as xe2x80x9cporous aluminaxe2x80x9d, without the modifier xe2x80x9cmacroxe2x80x9d, for convenience.
A gas impervious surface is needed on the ends of the porous ceramic tube to lock the membranes into the flowstream and to prevent the feed gas from bypassing around (i.e., short-circuiting) the internal microporous membrane, which would degrade the overall separation efficiency. At low temperatures, organic materials can be used to seal the porous surface, such as silicone, epoxy, acrylic, or other polymers. However, these organic materials are unsuitable for use at high temperatures, such as  greater than 200 C, at which point they decompose.
High temperature metallic braze alloys that wet ceramics can be used to seal the porous surface. However, high differential thermal stresses can be generated in the ceramic filter due to the large mismatch in the coefficient of thermal expansion (CTE) between the braze alloy (high CTE) and the ceramic (low CTE), leading to cracking and loss of sealing ability.
Commercially available ceramic glazes can be used to make the gas impermeable surface around the edges or ends of a porous ceramic filter. The ceramic glaze can withstand higher temperatures than organic sealants. The ceramic glazes generally have a closer CTE match to the ceramic filter media than metals, which reduces thermal mismatch stresses. However, commercially available glazes typically include low melting point compositions of mixed oxides, for example, Na2O, K2O, PbO, and CaO. Use of these mixed oxides makes them unsuitable for use at high temperatures (e.g., 500 C). Also, undesirable elements in these low-temperature ceramic glazes can disassociate and/or diffuse out from the glaze during the subsequent process of depositing or synthesizing the inorganic thin film microporous membrane (e.g., an inorganic zeolite membrane, which is deposited at from 90-200 C). These undesirable elements (or compounds) can interfere with the deposition/synthesis process, poison the membrane, and/or create defects in the microporous structure. These types of glazes also have a lower chemical resistance, which could partially dissolve in the high pH zeolite crystal-growing bath (the high temperatures used to grow the zeolites would increase the dissolution rate also).
The gas impermeable glaze preferably should be applied prior to growing the thin zeolite film. Because zeolite crystallization generally occurs under basic conditions, at high temperature, and sometimes with organic solvents, the glaze needs to be chemically stable. The glaze should not dissolve and become part of the reaction mixture during zeolite growth, which would interfere with crystal growth rates and affect the zeolite crystal""s pore size. Since synthesis of zeolite films can be performed at temperatures as high as 200 C, the glaze should be stable at temperatures greater than 200 C (and, preferably, greater than 500 C for use in hydrocarbon gas separation processes). The glazed coating needs to be sufficiently strong to permit the use of gas-tight, metallic compression-type seals to be made (e.g., Swagelock(trademark) type compression fittings) to the glaze. The glazed coating should be relatively thin, but not so thin as to crack or not provide a 100% xe2x80x9cpinholexe2x80x9d free (i.e.,  less than 3 Angstroms) hermetic seal.
A need remains, therefore, for a material and method for glazing and sealing the edges or ends of a porous surface, such as a ceramic filter media, with a material that is gas impervious, can withstand high temperatures, is chemically stable, mechanically strong, relatively thin, and can be applied prior to synthesizing inorganic thin film separation membrane, such as zeolite-based microporous membranes.
Against this background, the present invention was developed.